The present invention relates in general to signal processing systems, and more particularly, to a digitally programmable charge-transfer filter.
Charge-transfer devices are uniquely applicable to many analog signal processing functions because they are capable of operating directly with analog signals. One of the most important signal processing functions for which charge coupled devices can be used is the time delay of analog signals. A number of charge-transfer devices have been developed for signal processing, of which the transversal filter or multi-tap delay line has special importance. In addition to being a natural architecture for charge-transfer devices, it is a general purpose device in the sense that any desired finite impulse response can be obtained if tap locations and weights can be arbitrarily assigned. The structure in which taps weights and locations can not only be arbitrarily assigned but can also be electrically varied has even more significance in the context of digital processing, for such a device could be automatically programmed to perform a wide range of linear signal processing functions.
A transversal fitler can have an arbitrary impulse response of finite time duration and therefore can be used to implement a wide variety of linear filters. (Any system whose output is linearly related to the input is a linear filter). In this sense, a transversal filter can be thought of as the fundamental building block of linear systems. Since the impulse response of a charge-transfer transversal filter can be selected arbitrarily, these filters can be matched to any desired signal wave form of finite duration, in which case the filter is called a matched filter. Matched filters are used to detect a gain waveform in the presence of noise with optimum detection probability. Charge-transfer matched filters are useful, for example, in low data rate spread spectrum communication systems.
However, charge-transfer transversal filters can also be designed to achieve a particular spectral characteristic by frequency filtering. A linear phase bandpass filter can be constructed by selecting the impulse response of a transversal filter to be the Fourier transform of the desired frequency characteristic. One of the advantages of such filters is that their frequency characteristic scales with the clock frequency and by varying the clock frequency the filter can be tuned.
Another application of the charge-transfer transversal filter is that of auto or cross correlation. In the processing of electrical signals, it is often required to extract a signal from a noise background. This is often effected using auto correlation or cross correlation techniques. Such auto correlation is typically effected using devices generally referred to as matched filters, transversal filters and chirp filters. However, the charge-transfer transversal filter can be made by nondestructively sampling each charge packet in a charge coupled device delay line, multiplying the samples by weighting coefficients, then summing the results. Circuitry for performing the sampling, weighting and summing functions is integrated with the charge-transfer device in a single IC. Fixed weighting coefficient filters are factory programmable in the sense that the code or impulse response is determined by a single photomask which is used in the IC manufacture.
Many important analog signal processing functions, however, require variable weighting coefficient filters. Variable weighting coefficient filtering applications may be separated into categories: (1) convolution with an impulse response which is fixed for long periods of time but which must be varied slowly or changed frequently, and (2) convolution or correlation of two arbitrary waveforms.
One important application in the former category is matched filtering on a waveform which changes intermittently. Another important application in the former category is adaptive equalization in which the dispersion due to a changing transmission medium must be inverted in the receiver or MODEM.
Several approaches have been proposed to provide electronically variable weighting coefficients. In "CCD MNOS Devices For Programmable Analog Signal Processing and Digital Non-volatile Memory", N. H. White, D. R. Lampe, and J. L. Fagan, 1973 International Electron Devices meeting, Tech. Dig., PP. 130-133, MNOS transistors have been proposed as a means of varying filter weighting coefficients. The conductance of the respective transistor is programmed by the analog voltage applied to the gate. This approach has two limitations which hinders the cost effectiveness: (1) the MNOS circuitry introduces added processing complexity and corresponding added cost, and (2) the off chip circuitry required to program the MNOS conductance is formidable. In "Intracell Charge Transfer Structure for Signal Processing", J. J. Tiemann, W. E. Engeler, R. D. Baertsch, and D. N. Brown, IEE Transactions or Electron Device, ED-21, PP. 300-308, May 1974, an approach is presented for implementing a fully programmable transversal filter using a charge sloshing technique. This technique also suffers from the introduction of added processing complexity and correspondingly added costs.
Accordingly, an object of the present invention is to provide a digitally programmable charge-transfer filter.
Another object of the present invention is to provide a charge-transfer transversal filter having electronically variable tap weights.
Yet another object of the present invention is to provide an M-bit by N correlation digitally programmable filter comprised of a multiphase charge-transfer shift register.